The present invention relates to magneto-optical file memories, and more particularly to a magneto-optical file memory capable of rewriting information.
As a rewritable magneto-optical file memory, there has been known a magneto-optical disk device wherein a storage medium having a vertical magnetic film is irradiated with a laser beam and has the direction of magnetization thereof reversed by the resulting absorption of photon energy, thereby to write and/or erase information, and the rotation of polarizable face of reflected light from the storage medium is detected, thereby to reproduce information (refer to, for example, Y. Togami et al, "Amorphous thin film disk for magneto-optical memory," SPIE, vol. 329, Optical Disk Technology (1982), pp. 208-214). For reversing the direction of magnetization in the device of this type, there are a writing form for Curie point and a writing form for compensatory temperature. Both of them make use of changes in the coercive force and magnetization of the storage medium based on temperatures. Here, the principles of operations for writing and erasing information will be described with reference to FIGS. 1 and 2 by taking the Curie-point writing system as an example.
FIG. 1 illustrates the principle of the writing operation. (a) shows unidirectional magnetization Ms before storage in a magnetic film. The state in which the magnetic film is magnetized in one direction in this manner is set as the state of initial magnetization. (b) shows that, when the surface of the magnetic film is irradiated with a laser beam 1 in the form of a spot, the minute part is heated to or above the temperature of the Curie point until the coercive force and the magnetization become null. (c) shows a state resulting when a magnetic field for writing Hb (about 100 Oe.) is applied to the state (b) in the direction opposite to that of the magnetization Ms. When the temperature has fallen, reversed magnetization arises owing to the sum between the writing magnetic field Hb and an opposite magnetic field Hd based on the magnetization Ms (actual magnetic field for writing=Hb+Hd). (d) shows a reversed magnetization state (written domain) stable in the cooled condition of the minute part (at the room temperature). The writing of information is performed in such sequence.
Next, the principle of the erasing operation is illustrated in FIG. 2. (a) shows the state (d) explained in FIG. 1. (b) shows the state (the same as (b) in FIG. 1) in which the written minute part is irradiated with a laser beam in the form of a spot and heated to or above the temperature of the Curie point. (c) shows a state resulting when a magnetic field for erasing Hb.sub.1 (about 200 Oe.) is applied to the state (b) in the same direction as that of the magnetization Ms. At this time, the magnetization of the magnetic film has the same sense as that of the surrounding magnetization Ms owing to the difference between the erasing magnetic field Hb.sub.1 and the opposite magnetic field Hd resisting it (actual magnetic field for erasing=Hb.sub.1 -Hd). (d) shows the state of the magnetization Ms stable in the cooled condition of the minute part (at the room temperature). The erasing is performed in the above way.
As thus far described, information is written and erased by the magnetization reversal (of the surface) of the magnetic film of the storage medium. In this regard, in order to record or erase information precisely at the predetermined parts of the surface of the magnetic film, the so-called auto-focusing is required in which a laser beam spot as predetermined is projected on the surface of the magnetic film of the storage medium while following up the motion of the storage medium. To this end, an actuator for the fine adjustments of a lens is usually used.
With the prior-art magneto-optical file memory, however, the diameters of written domains become unequal, and an imperfect erased state is involved, resulting in the disadvantage that the writing, reproduction and erasing of information cannot be precisely conducted.